The term rudimentary magic is used to refer to a specific brand of lesser magic employed by hags and trolls. It seems to be a simpler, much tamer form of dark magic. Hags can use it to brew potions.
Neither hags nor trolls seem to frequently resort to it in their activities — hags more often tend to use their cunning, reserving their magic for brewing potions, while trolls make do with their prodigious brutish strength — which seems to suggest that this kind of magic is quite mild.
Behind the scenes
- The existence of this kind of magic is hinted at in the third W.O.M.B.A.T. featured in J. K. Rowling's official site. Question 14 (Part Four - Dark Magic) is "Which of the following is FALSE?", and one of the possible answers is "Hags have only rudimentary magic, similar to that observed in trolls". As one of the other answers is known for certain to be false ("Patronuses vary in strength according to which animal's form they take" — Wonderbook: Book of Spells stresses that there is no correlation between the form of the conjured animal and the strength of the Patronus), then all of the other answers must be true, including the one about trolls' and hags' rudimentary magic.